Method for performing handoff in wireless network

ABSTRACT

A method for minimizing handoff latencies when a handoff is performed in a wireless network. An access point (AP) or base station associated to a current wireless station (STA) allows information required for a reassociation to the STA to be propagated to handoff-capable neighboring APs or base stations. When the STA moves, a neighboring AP or base station performs the reassociation to the STA on the basis of context. When a handoff procedure is performed, the time taken to receive context of a corresponding STA is reduced, such that a fast handoff can be implemented.

PRIORITY

This application claims priority to a provisional application entitled“A METHOD FOR FAST AND SECURE WIRELESS LOCAL AREA NETWORK HANDOFFS”,filed in the United States Patent and Trademark Office on Nov. 8, 2002and assigned Ser. No. 60/425,109, the contents of which are herebyincorporated by reference.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.60NANB1D0113 awarded by the National Institute of Standards andTechnology, and under Contract No. MDA90402C0428 awarded by the NationalSecurity Agency. The U.S. Government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for performing a handoff in afast and secure wireless network, and more particularly to a method forminimizing handoff latencies.

2. Description of the Related Art

Conventionally, a local area network (LAN) is a collection of personalterminals, main frames and workstations coupled to a communication linkwithin a distance of 300 meters or less. The LAN is a high-speedcommunication network for allowing employees in a company to be aware ofinformation, i.e., a distance in which an electric current or radio wavesignal can be correctly transferred between the personal terminals, tocommonly and most effectively use equipment installed in the company'sbuilding. As LANs, wired networks for directly transferring anelectrical signal through the communication link have been initiallyused. The trend has been to replace the wired networks with wirelessnetworks for transferring a signal using a radio wave in accordance withthe development of wireless protocols. LANs based on these wirelessnetworks are referred to as wireless local area networks (WLANs). WLANsare based on Institute of Electrical and Electronics Engineers (IEEE)802.11. IEEE 802.11-based WLANs have seen immense growth in the last fewyears. It is predicted that the IEEE 802.11-based WLANs will be rapidlydeveloped in the future because of an advantageous effect of convenientnetwork connectivity.

IEEE 802.11 allows for two operating modes, i.e., an ad hoc mode and aninfrastructure mode, in relation to a media access control (MAC) layer.In the ad hoc mode, two or more wireless stations (STAs) recognize eachother and establish a peer-to-peer communication without any existinginfrastructure. Meanwhile, in the infrastructure mode, there is a fixedentity called an access point (AP) that bridges all data between theSTAs associated with the AP. The AP and the STAs associated with the APform a basic service set (BSS) communicating on the unlicensed radiofrequency (RF) spectrum.

FIG. 1 is a view illustrating the architecture of a conventionalwireless local area network (WLAN) for supporting the infrastructuremode.

Referring to FIG. 1, a plurality of access points (APs) 120 a and 120 bare connected through one distribution system (DS) 110. The DS 110 isimplemented with a wired network. A communication path is formed betweenthe plurality of APs 120 a and 120 b. The plurality of APs 120 a and 120b form constant service areas, and serve as bridges between STAs 130 a,130 b, 130 c and 130 d and the DS 110. One AP and the STAs associatedwith the AP form a basic service set (BSS). In other words, a unique BSSis formed on an AP-by-AP basis, and service is provided on a BSS-by-BSSbasis. A plurality of BSSs formed by the APs 120 a and 120 b can beextended to extended service sets (ESSs). The STAs 130 a, 130 b, 130 cand 130 d must undergo an authentication procedure to access the WLANthrough the APs 120 a and 120 b to which the STAs 130 a, 130 b, 130 cand 130 d belong. In other words, the STAs 130 a, 130 b, 130 c and 130 dare permitted to access the network through the authenticationprocedure. There is provided state information required so that the STAs130 a, 130 b, 130 c and 130 d can access the network according to theauthentication procedure. The state information contains encryptioninformation (based on an encryption code) used to transfer data to theDS 110.

In the WLAN based on the architecture shown in FIG. 1, a wirelessstation (STA) has mobility and hence can move from one BSS to anotherBSS. In this case, a handoff is required so that service being receivedfrom the one BSS can be continuously provided to the STA by another BSS.An AP to which the STA had physical layer connectivity prior to thehandoff is referred to as a “prior-AP”, while a new AP to which the STAacquires physical layer connectivity after the handoff is referred to asa “post-AP”.

The conventional handoff procedure refers to the mechanism or sequenceof messages exchanged between the APs and the STA. In the conventionalhandoff procedure, physical layer connectivity and state informationmust be transferred from one AP to another AP with respect to the STA inconsideration. The handoff is a physical layer function carried out byat least three participating entities, i.e., an STA, a prior-AP and apost-AP. The state information that is transferred typically consists ofthe client credentials (which allow the STA to gain network access) andsome accounting information. An operation for transferring the stateinformation can be performed by an inter access point protocol (IAPP).For an IEEE 802.11 network that has no access control mechanism, therewould be a nominal difference between a completion association and ahandoff/reassociation. Looking at it another way, handoff latency wouldbe strictly greater than association latency as there is an additionalinter-access point communication delay involved.

Logical steps based on the handoff procedure are classified into adiscovery phase and a reauthentication phase.

1. Discovery Phase: Attributing to mobility, the signal strength and thesignal-to-noise ratio of a signal from the STA's current AP (orprior-AP) might degrade and cause it to initiate a handoff. At thispoint, the STA might not be able to communicate with its current AP (orprior-AP). Thus, the STA needs to find potential APs in range toassociate to. This is accomplished by a MAC layer function (or scanfunction). During a scan, the STA listens for beacon messages sent outperiodically by APs at a rate of 10 ms, on assigned channels. Thus, theSTA can create a priority list, i.e., a list of APs prioritized by thereceived signal strength. Two kinds of scanning methods defined in thestandard are based on an active mode and a passive mode. As the namessuggest, in the active mode, apart from listening to beacon messages(which is passive), the STA sends additional probe broadcast packets oneach channel and receives responses from APs. Thus, the STA activelysearches or probes for potential APs.

2. Reauthentication Phase: The STA sends a reauthentication request topotential APs according to the priority list in the above-describeddiscovery phase. The reauthentication phase typically involves anauthentication and a reassociation to the post-AP. The reauthenticationphase involves the transfer of credentials and other state informationfrom the prior-AP. As mentioned earlier, this can be achieved through aprotocol such as the IAPP. The reauthentication phase includes anauthentication phase and a reassociation phase.

FIG. 2 is a view illustrating a handoff procedure in the conventionalWLAN. It is assumed in FIG. 2 that the discovery phase is performed inthe active mode. The handoff procedure shown in FIG. 2 is divided into aprobe phase 210 and a reassociation phase 220.

Referring to FIG. 2, a wireless station (STA) sensing the need for thehandoff transmits a probe request message to a plurality of unspecifiedAPs at step 212. The probe request message is defined as information forasking each AP whether or not the handoff can be successfully performed.Upon receiving the probe request message, the APs transmit proberesponse messages to the STA at step 214. Here, the fact that certainAPs have received the probe request message means that the APs areadjacent to the STA. Thus, the APs capable of receiving the proberequest message are determined to be potential APs. The STA repeatedlyperforms the above-described operation on a channel-by-channel basis.

On the other hand, the STA performs the reassociation phase 220according to priorities of the potential APs registered in a prioritylist created in the discovery phase. The STA transmits a reassociationrequest message to a new AP at step 222. In response to thereassociation request message, the new AP performs an inter access pointprotocol (IAPP) procedure with other APs, i.e., a prior AP of the STA,at step 230. Through the IAPP procedure, the new AP receives credentialsand other state information assigned to the STA. Then, the new APtransmits, to the STA, a reassociation response message to thereassociation request message at step 224.

As described above, the conventional handoff procedure starts when theSTA transmits a probe request message and ends when the STA receives areassociation response message. During the handoff procedure, threetypes of delay are incurred as in the following. The three types ofdelay include a probe delay incurred in the discovery phase, anauthentication delay incurred in the authentication phase and areassociation delay incurred in the reassociation phase.

1. Probe Delay: Messages transmitted for an active scan at the probephase 210 shown in FIG. 2 are probe messages. The latency for thisprocess is referred to as a probe delay. The STA transmits a proberequest message and waits for responses from APs on each channel. Thetime during which the STA waits on a particular channel after sendingthe probe request message corresponds to probe-wait latency. This isdetermined to be a time difference between subsequent probe requestmessages. Here, the time is subsequent between PROBE REQUEST MESSAGES ondiffering channels. According to the above procedure, it has been foundthat the traffic on the channel and the timing of probe responsemessages affect the probe-wait time.

2. Authentication Delay: This is the latency (not shown in FIG. 2)incurred during which authentication frames are exchanged.Authentication consists of two or four consecutive frames depending onthe authentication method used by the AP. Some wireless networkinterface cards (NICs) try to initiate a reassociation prior to theauthentication, which causes an additional delay in the handoff process.

3. Reassociation Delay: This is the latency incurred during whichreassociation frames are exchanged in the reassociation phase 220 shownin FIG. 2. If an authentication process is successful, the STA sends areassociation request frame to the AP, receives a reassociation responseframe, and completes the handoff. Where the IAPP procedure between a newAP and other APs is additionally required, the reassociation delay willfurther increase.

According to the above, messages during the probe delay form thediscovery phase, while the authentication and reassociation delays formthe reauthentication phase. Apart from the latencies discussed above,there will potentially be a bridging delay caused by the time taken forthe MAC address updates to Ethernet switches which form the distributionsystem (i.e., the backbone Ethernet). It can be seen that many latenciesare incurred while a handoff between an STA and APs is performed in theconventional WLAN. There are problems in that the latencies not onlyaffect the quality of service (QoS) but also disable high-speed roaming.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of the present invention to provide amethod for minimizing handoff latencies.

It is another aspect of the present invention to provide a method fortransferring state information of a corresponding wireless station (STA)to access points (APs) before a handoff is performed.

It is yet another aspect of the present invention to provide a handoffmethod capable of eliminating a tunneling procedure between aprior-access point (AP) and a post-AP and a procedure of transferringstate information of a corresponding wireless station (STA) through thetunneling procedure.

It is still another aspect of the present invention to provide a methodfor generating a neighborhood graph needed to send state information ofa wireless station (STA) to potential access points (APs).

It is still yet another aspect of the present invention to provide amethod for propagating state information of a wireless station (STA) toneighboring access points (APs) on the basis of a neighborhood graph.

To achieve the above and other aspects of the present invention, thereis provided a handoff method for a station in a wireless network. Themethod includes: outputting a re-association request to an access pointin the wireless network to access the wireless network through theaccess point; and receiving a re-association response responsive to there-association request from the access point, said access point havinginformation including context of the station prior to receiving there-association request.

Additionally, there is provided a handoff method for an access point ina wireless network through which a station accesses the wirelessnetwork. The method includes outputting a re-association responseresponsive to a re-association request from the station, said accesspoint having information including context of the station prior toreceiving the re-association request.

Additionally, there is provided another handoff method for an accesspoint in a wireless network through which a station accesses thewireless network. The method includes: receiving a re-associationrequest from the station; determining whether the access point includescontext of the station prior to receiving the re-association request;and outputting a re-association response responsive to there-association request where the access point includes the context priorto receiving the re-association request.

Additionally, there is provided an access point through which a stationaccesses a wireless network. The access point includes a memory forstoring information including context of the station, wherein the accesspoint outputs a re-association response responsive to a re-associationrequest from the station where the storage includes the context prior toreceiving the re-association request.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a view illustrating the architecture of a conventionalwireless local area network (WLAN);

FIG. 2 is a view illustrating a handoff procedure in the conventionalWLAN;

FIGS. 3A and 3B are views illustrating an operation for generating aneighborhood graph in accordance with an embodiment of the presentinvention;

FIG. 4 is a conceptual view illustrating the handoff procedure inaccordance with the embodiment of the present invention;

FIG. 5 is a view illustrating the handoff procedure in a wireless localarea network (WLAN) in accordance with the embodiment of the presentinvention; and

FIG. 6 is a flow chart illustrating operations of access points (APs) inaccordance with the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be described indetail with reference to the annexed drawings. In the followingdescription, the present invention proposes the preferred embodiment toachieve the above and other objects. However, other embodiments of thepresent invention can be drawn from the following description of thepresent invention.

According to an aspect of the present invention, a proactive cachingtechnique is adopted to reduce a reassociation delay. In order for theproactive caching technique to be adopted, a procedure of propagatingstate information of a corresponding wireless station (STA), i.e.,context, from a prior-access point (AP) to potential APs is performedirrespective of a handoff process. The potential APs are a set of APswith which the STA can associate from the prior AP of the STA. In orderfor the context of the STA to be sent to the potential APs as describedabove, the potential APs may be managed on each AP. For this, the APsmay generate and manage a neighborhood graph. The neighborhood graphdefines connection relationships between the potential APs and the priorAP in the handoff process.

According to another aspect, as illustrated in FIG. 3B, a data structuremay be provided for containing information on one or more potential APsfor each of the APs. In this case, such a structure may be provided in amedium accessible by each of the APs. Methods for generating theneighborhood graph and a handoff procedure based on the proactivecaching technique using the neighborhood graph will be described indetail herein below.

1. Generation of Neighborhood Graph

In accordance with the present invention, a neighborhood graph is formedby the arrangement of APs configuring a wireless local area network(WLAN). As potential APs corresponding to each of the APs configuringthe WLAN are different, the generation of the neighborhood graph isachieved on an AP-by-AP basis. Three types of neighborhood graphgeneration methods are disclosed below. The first generation methodallows a manager to manually generate the neighborhood graph. The firstgeneration method allows the manager to configure and registerneighborhood graphs on the AP-by-AP basis according to the arrangementof the APs and also allows the manager to update a neighborhood graphwhen the arrangement of APs is changed therein. The second generationmethod allows the manager to register the first neighborhood graph andalso allows the neighborhood graph to be automatically changed when thearrangement of the APs is changed therein. The third generation methodallows neighborhood graphs to be automatically generated on an AP-by-APbasis. In the third generation method, a handoff is performed on thebasis of an existing handoff procedure to generate or update aneighborhood graph. In other words, a procedure of confirming connectionrelationships on an AP-by-AP basis is performed in the third generationmethod. For example, where the STA located at an AP_A tries to firstperform a handoff process to an AP_B by which no handoff for the STA waspreviously performed, the AP_B performs an inter-access point protocol(IAPP) procedure to receive context corresponding to the STA from theAP_A. Then, AP_A and AP_B confirm the existence of the interconnectionrelationship therebetween for the handoff, such that a correspondingneighborhood graph can be generated or updated. After the neighborhoodgraph is updated, the handoff can be performed with respect to the STAthat desires to move from AP_A to AP_B or from AP_B to AP_A without theIAPP procedure.

A physical path connected between APs and a distance between the APsshould be considered so that any one of the three types of generationmethods can generate a neighborhood graph. In other words, the APsconfiguring the WLAN must be able to be physically connected to eachother without going through any other AP so that connectionrelationships can be formed on the basis of the neighborhood graph.Furthermore, two APs physically connected to each other should be withina threshold distance range. Where the two APs are far away from eachother, a handoff may be performed according to an initial procedure forallowing a new AP to support communication.

An example of generating a neighborhood graph to be applied inaccordance with an embodiment of the present invention will now bedescribed in detail.

FIG. 3A is a view illustrating an exemplary arrangement of APsconfiguring the WLAN to which an embodiment of the present invention isapplied; and FIG. 3B is a view illustrating an exemplary neighborhoodgraph capable of being generated by the arrangement of APs shown in FIG.3A.

As shown in FIG. 3A, an AP_C is installed in a closed space with onegateway. Thus, a path in which the STA located at AP_C can move isdefined by an AP_B. This means that only a handoff process between AP_Cand AP_B can be performed with respect to the STA located at AP_C. TheSTA located at AP_B can move not only to AP_A, AP_D and AP_E but also toAP_C, which are installed at passages (as physical connections). Inother words, the STA located at AP_B allows a handoff process to beperformed between AP_B and all other APs shown in FIG. 3A. APs to whichthe STA located at AP_A can directly associate without going through anyother AP are defined by AP_B and AP_E. Thus, the STA located at AP_Aallows a handoff process to be performed between AP_A and AP_B or AP_E.The STA located at AP_E can directly associate to all APs other thanAP_C among APs shown in FIG. 3A. This means that the STA located at AP_Eallows a handoff process to be performed between AP_E and any AP exceptfor AP_C. APs to which the STA located at AP_D can directly associatewithout going through any other AP are defined by AP_B and AP_E. Thus,the STA located at AP_D allows a handoff process to be performed betweenAP_D and AP_B or AP_E. A reason why a handoff between AP_D and AP_A isnot permitted is because a distance between AP_D and AP_A is outside apredetermined threshold distance range.

FIG. 3B shows a neighborhood graph generated by the connectionrelationships between the above-described APs. The neighborhood graphshown in FIG. 3B shows the connection relationships between all APsconfiguring the WLAN. In accordance with the present invention, each APonly needs to recognize potential APs capable of being associatedtherewith. For example, AP_A only needs to recognize AP_B and AP_E asits potential APs, while AP_B only needs to recognize AP_A, AP_C, AP_Dand AP_E as its potential APs. As described above, while notillustrated, a neighborhood graph on each AP may be generated by themanager or can be automatically generated according to an existinghandoff procedure.

An operation for allowing each AP to automatically generate theneighborhood graph will now be described. Upon receiving a reassociationrequest message from a wireless station (STA), an arbitrary APdetermines whether temporarily stored context corresponding to the STAis present. At this point, the arbitrary AP becomes a post-AP for theSTA. The fact that the context is present means that a neighborhoodgraph with a prior-AP from which the STA moves is formed. On the otherhand, if the context is not present, it can be determined that theneighborhood graph with the prior-AP from which the STA moves is notformed. In this case, the post-AP receives the context corresponding tothe STA from the prior-AP through the existing IAPP, updates theneighborhood graph and forms a connection with the prior-AP. Inaccordance with a handoff procedure of the present invention, thehandoff can be performed with respect to the STA that moves from theprior-AP after the connection is formed.

2. Proactive Caching Technique

In a proactive caching technique according to an embodiment of thepresent invention, each AP recognizes its potential APs. Context of theSTA belonging to the AP is sent to the potential APs. Even though theSTA belonging to an arbitrary AP moves to any AP connected to thearbitrary AP, the time required for a reassociation phase in the handoffprocedure is minimized. That is, the proactive caching technique isbased on some locality principle of mobility. In this environment, areassociation pattern of the STA will be the sequence of APs that theSTA gets associated with in a given interval of time.

Proactive caching techniques for reducing a reassociation delay inaccordance with the embodiment of the present invention will now bedescribed in detail with reference to FIG. 4. FIG. 4 is a conceptualview illustrating a handoff procedure based on the proactive cachingtechnique in accordance with the embodiment of the present invention.Here, it is assumed that a wireless station (STA) moves from an AP_A toan AP_B.

Referring to FIG. 4, the STA sends an association/reassociation requestto AP_A at step 1. AP_A performs different operations according towhether the association or reassociation request is received from theSTA.

When the association request is received, AP_A performs anauthentication process for the STA on the basis of a typical initialauthentication procedure. If the authentication process is completed,AP_A sends, to the STA, a response message to the association request.

When the reassociation request is received, AP_A performs differentoperations according to whether or not context corresponding to the STAhas been temporarily stored. If the context corresponding to the STA hasbeen temporarily stored, AP_A sends a response message to the STA inresponse to the reassociation request. On the other hand, if the contextcorresponding to the STA has been not temporarily stored, AP_A receivesthe context from an AP at which the STA was previously located throughthe typical IAPP procedure. Then, the response message to thereassociation request is sent to the STA. The STA performs communicationwith AP_A by receiving the response message from AP_A.

On the other hand, AP_A transfers the context, such as security context,corresponding to the STA to AP_B indicating a potential AP in a handoffat step 2. Only one AP is shown as the potential AP in FIG. 4. However,where a plurality of APs are present as potential APs, the context ispropagated to the plurality of APs. AP_B stores the context transferredfrom AP_A in a cache. After moving to AP_B through a predetermined path,the STA sends a reassociation request to AP_B at step 3. In response tothe reassociation request, AP_B performs communication with the STAaccording to the context previously transferred from AP_A. In otherwords, the reassociation between AP_B and the STA is performed accordingto the context. Thus, the present invention reduces a time delayincurred during the IAPP procedure and hence improves a communicationrate.

An embodiment of the present invention employs proactive cachingtechnique in which context of a corresponding STA can be provided to atleast one predicted AP to which the STA moves. In other words, in orderfor the proactive caching technique to be applied, an operation fortransferring context of a corresponding STA from a prior-AP to a post-APis performed. Furthermore, each AP is able to predict information aboutpotential post-APs so that the proactive caching technique may beapplied. This has been described above in relation to the neighborhoodgraph.

In accordance with an embodiment of the present invention, a method ofreducing a reassociation delay using the proactive caching techniquewill now be described in detail with reference to FIG. 5. FIG. 5 is aview illustrating a handoff procedure using the proactive cachingtechnique in the WLAN in accordance with the embodiment of the presentinvention.

Referring to FIG. 5, context of a corresponding STA is transferred froma prior-AP to a post-AP before a reassociation process for the handoffis performed. In FIG. 5, it is assumed that AP_A is the prior-AP andAP_B is the post-AP. Furthermore, it is assumed that the context of thecorresponding STA is already temporarily stored.

Referring to FIG. 5, the STA sends a reassociation request message toAP_A at step 501. At this time, AP_A may already have stored the contextof the STA using the proactive caching technique. Otherwise, if AP_A hasnot stored the context of the STA, AP_A can receive the context of theSTA from the WLAN through the typical authentication procedure orreceives the context of the STA from an AP at which the STA waspreviously located through the IAPP procedure. AP_A transmits areassociation response message to the STA on the basis of thetemporarily stored context corresponding to the STA at step 503. Then,AP_A propagates the temporarily stored context to a potential AP, i.e.,AP_B at step 505. At this time, information of the potential AP can beobtained from the above-described neighborhood graph. It is assumed thatthe number of potential APs is one as shown in FIG. 5, but a pluralityof potential APs can be present. If the multiple potential APs arepresent, AP_A propagates the context of the STA to the plurality ofpotential APs. AP_B temporarily stores the context corresponding to theSTA propagated from AP_A.

At the time of the need for a handoff to AP_B, the STA sends areassociation request message to AP_B at step 507. Upon receiving thereassociation request message, AP_B determines whether the temporarilystored context corresponding to the STA is present. If the temporarilystored context corresponding to the STA is present in AP_B, AP_Btransmits a reassociation response message to the STA on the basis ofthe context at step 509. As authentication is completed between the STAand AP_B, communication between the STA and AP_B is enabled. Since theAP_B includes the context of the STA, further/another authentication mayalso be readily performed.

Where the proactive caching technique is applied as described above, astate in which each AP cannot store context propagated from neighboringAPs may be incurred due to insufficient storage, for example, cachecapacity. In this case, the AP sequentially deletes the oldest contextsso that newly propagated context can be stored.

3. Description of Operation in Accordance With the Present Invention

An operation of the AP when a handoff procedure is performed inaccordance with an embodiment of the present invention will now bedescribed in detail with respect to FIG. 6. A procedure of receiving andstoring context received from neighboring APs, a procedure of performingan operation in response to an association request, and a procedure ofperforming an operation in response to a reassociation request will nowbe described with reference to FIG. 6.

Referring to FIG. 6, the AP determines whether context corresponding toa specific wireless station (STA) is received from handoff-capableneighboring APs that are managed by a neighborhood graph at step 610.Upon receiving the context corresponding to the specific STA, the APproceeds to step 612, and stores the received context in its own cache.Although, FIG. 6 illustrates the steps for receiving and storing thesecurity context, these steps are not necessarily required to performthe following steps 614-626.

Accordingly, the AP determines, at step 614, whether an associationrequest has been received from the STA, and determines, at step 616,whether a reassociation request has been received from the STA. If theassociation request has been received from an arbitrary STA, the APproceeds to step 618 and performs a typical authentication procedurewith an authentication server provided in a wireless network. Then, theAP configures context corresponding to the STA and stores the configuredcontext in its own cache. In step 616, if the reassociation request hasbeen received, the AP determines that the STA has moved from another AP.Then, the AP proceeds to step 620 and determines whether contextcorresponding to the STA stored in the internal cache is present. If thecontext corresponding to the STA is not present in the internal cache,the AP proceeds to step 622. At the above step 622, the AP performs atypical IAPP procedure, and obtains the context corresponding to the STAfrom another AP at which the STA was previously located. If the APrecognizes another AP at which the STA was previously located, the IAPPprocedure is performed only for the already recognized AP.

When the AP proceeds from the above step 618, 620 or 622 to step 624,the AP sends a response message to the STA. The response messagecorresponds to the association/reassociation request. Then, the APproceeds to step 626 after sending the response message, the AP refersto a neighborhood graph managed thereby and propagates the context of acorresponding STA to neighboring APs. This is to implement a fasthandoff when a corresponding STA moves to any neighboring AP.

As apparent from the above description, the present invention canprovide a method for simplifying a handoff procedure in a wireless localarea network (WLAN), reducing a reassociation delay, and enabling awireless station (STA) to quickly communicate with an access point (AP)to which the STA moves. Furthermore, the method in accordance with thepresent invention can provide not only secure quality of service butalso high-speed roaming service.

Furthermore, the present invention is applicable to all wirelesscommunication systems and technologies, and as such may be utilized withCDMA, TDMA, FDMA, IMT, GSM, etc. systems and equipment, as well as IEEE802.11 technology and equipment. APs as described above are analogous tobase stations in telecommunication systems, while STAs are analogous tomobile terminals or stations.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope of the invention. Therefore,the present invention is not limited to the above-described embodimentsand drawings.

What is claimed is:
 1. A handoff method for a station in a wirelessnetwork, the method comprising: outputting a first reassociation requestto a prior access point in the wireless network; receiving, from theprior access point, a first reassociation response in response to thefirst reassociation request, the prior access point transmitting acontext of the station to a post access point in the wireless network inresponse to the first reassociation response being transmitted to thestation, the context allowing the station to directly reassociate withthe post access point; outputting, to the post access point, a secondreassociation request to begin handoff to access the wireless networkthrough the post access point; and receiving, directly from the postaccess point, a second reassociation response in response to the secondreassociation request, when the post access point is determined to havethe context of the station prior to receiving the second reassociationrequest initiating the handoff procedure to the post access point,wherein the context is transmitted from the prior access point to thepost access point based on a neighboring graph including a list ofaccess points within a predetermined path of the station.
 2. The methodaccording to claim 1, wherein the post access point receives, from theprior access point, the context of the station in response to thestation associating or reassociating with the prior access point.
 3. Themethod according to claim 1, wherein the station is a mobile terminal.4. The method according to claim 1, wherein the post access point havingthe context of the station is indicative of the station beingauthenticated.
 5. The method according to claim 1, further comprisingauthenticating the station using the context of the station.
 6. Anon-transitory computer-readable medium comprising computer-executableinstructions for performing the operations comprising: outputting afirst reassociation request to a prior access point in the wirelessnetwork; receiving, from the prior access point, a first reassociationresponse in response to the first reassociation request, the prioraccess point transmitting a context of the station to a post accesspoint in the wireless network in response to the first reassociationresponse being transmitted to the station, the context allowing thestation to directly reassociate with the post access point; outputting,to the post access point, a second reassociation request to beginhandoff to access the wireless network through the post access point;and receiving, directly from the post access point, a secondreassociation response in response to the second reassociation requestwhen said the post access point is determined to have the context of thestation, prior to receiving the second reassociation request initiatingthe handoff procedure to the post access point, wherein the context istransmitted from the prior access point to the post access point basedon a neighboring graph including a list of access points within apredetermined path of the station.
 7. A handoff method for a post accesspoint in a wireless network through which a station accesses thewireless network, the method comprising: receiving a context of thestation from a prior access point, the prior access point transmittingthe context to the post access point in response to the prior accesspoint transmitting a first reassociation response to the station, andthe context allowing the station to directly reassociate with the postaccess point; receiving a reassociation request from the station; andoutputting, directly to the station, a second reassociation response inresponse to the reassociation request, when the post access point isdetermined to have the context of the station prior to receiving thereassociation request initiating the handoff method to the post accesspoint, wherein the prior access point comprises a neighborhood graphincluding a list of access points within a predetermined path of thestation, and the prior access point transmits the context to the postaccess point based on the neighborhood graph.
 8. The method according toclaim 7, further comprising outputting the context of the station to oneor more access points of the wireless network that are fit for handoffof the station thereto from the post access point in response toreassociation of the station to the post access point.
 9. The methodaccording to claim 8, wherein the outputting of the informationincluding the context comprises: accessing a neighborhood graphcomprising information on the one or more access points; and propagatingthe context according to the information accessed from the neighborhoodgraph.
 10. The method according to claim 9, wherein the information onthe one or more access points comprises a connection relationshipbetween the post access point and the one or more access points.
 11. Themethod according to claim 9, wherein the neighborhood graph is providedin one of the post access point and a server of the wireless network.12. The method according to claim 7, further comprising: forming achannel with the prior access point of the station when it is determinedthat the information including the context of the station is not presentin the prior access point when the reassociation request is received;receiving the information including the context via the channel from theprior access point; outputting the reassociation response responsive tothe reassociation request in response to receiving information includingthe context; and outputting information including the context of thestation to one or more access points of the wireless network that arefit for handoff of the station thereto from the post access point. 13.The method according to claim 12, wherein the information including thecontext is received from the prior access point through an inter accesspoint protocol procedure.
 14. The method according to claim 7, furthercomprising: forming a channel with a prior access point of the stationwhen it is determined that the information including the context of thestation is not present in the prior access point when the reassociationrequest is received; receiving the information including the context viathe channel from the prior access point; outputting the re-associationresponse responsive to the reassociation request in response toreceiving information including the context; and registering the prioraccess point as fit for handoff of the station thereto from the postaccess point in a neighborhood graph provided in one of the access pointand a server of the wireless network accessible by the post accesspoint.
 15. The method according to claim 7, further comprising deletingan oldest context in response to insufficient storage capacity of thepost access point to store the context received from the prior accesspoint.
 16. A non-transitory computer-readable medium comprisingcomputer-executable instructions for performing the operation of:receiving a context of a station from a prior access point in a wirelessnetwork, the prior access point transmitting the context to a postaccess point in response to the prior access point transmitting a firstreassociation response to the station, and the context allowing thestation to directly reassociate with the post access point; receiving areassociation request from the station; and outputting, directly to thestation, a second reassociation response in response to thereassociation request, when the post access point is determined to havethe context of the station prior to receiving the reassociation requestinitiating the handoff operation to the post access point, wherein theprior access point comprises a neighborhood graph including a list ofaccess points within a predetermined path of the station, and the prioraccess point transmits the context to the post access point based on theneighborhood graph.
 17. A handoff method for an post access point in awireless network through which a station accesses the wireless network,the method comprising: receiving a context of the station from a prioraccess point in the wireless network, the prior access pointtransmitting the context to the post access point in response to theprior access point transmitting a first reassociation response to thestation, the context allowing the station to directly reassociate withthe post access point; receiving a reassociation request directly fromthe station to begin handoff of the station to the post access point;and determining whether the post access point includes the context ofthe station, prior to receiving the reassociation request; andoutputting, directly to the station, a second reassociation response inresponse to the reassociation request, when the post access point isdetermined to have the context prior to receiving the reassociationrequest initiating the handoff procedure to the post access point,wherein the prior access point comprises a neighborhood graph includinga list of access points within a predetermined path of the station, andthe prior access point transmits the context to the post access pointbased on the neighborhood graph.
 18. The method according to claim 17,wherein the post access point having the context of the station isindicative of the station being authenticated.
 19. The method accordingto claim 17, wherein the context of the station is information forauthenticating the station.
 20. A non-transitory computer-readablemedium comprising computer-executable instructions for performing theoperations of: receiving a context of a station from a prior accesspoint in the wireless network, the prior access point transmitting thecontext to a post access point in response to the prior access pointtransmitting a first reassociation response to the station, and thecontext allowing the station to directly reassociate with the postaccess point; receiving a reassociation request directly from thestation to begin handoff of the station to the post access point;determining whether the post access point includes the context of thestation; and outputting, directly to the station, a second reassociationresponse in response to the reassociation request, when the post accesspoint is determined to have the context prior to receiving thereassociation request initiating the handoff procedure to the postaccess point, wherein the prior access point comprises a neighborhoodgraph including a list of access points within a predetermined path ofthe station, and the prior access point transmits the context to thepost access point based on the neighborhood graph.
 21. A post accesspoint through which a station accesses a wireless network, the postaccess point comprising: an input configured to receive a context of thestation from a prior access point, and to receive a reassociationrequest directly from the station to begin handoff of the station to thepost access point, the prior access point transmitting the context tothe post access point in response to the prior access point transmittinga first reassociation response to the station, the context allowing thestation to directly reassociate with the post access point; a storage tostore the context of the station prior to receiving the reassociationrequest initiating the handoff procedure to the post access point; andan output to output, directly to the station, a second reassociationresponse in response to the reassociation request, when the storage isdetermined to have the context prior to receiving the reassociationrequest, wherein the prior access point comprises a neighborhood graphincluding a list of access points within a predetermined path of thestation, and the prior access point transmits the context to the postaccess point based on the neighborhood graph.
 22. The access pointaccording to claim 21, wherein the post access point determines whetherthe storage includes the context in response to the reassociationrequest.
 23. The access point according to claim 21, wherein the postaccess point further outputs information including the context of thestation to one or more access points of the wireless network that arefit for handoff of the station thereto from the post access point inresponse to reassociation of the station to the post access point. 24.The access point according to claim 23, wherein the post access pointaccesses a neighborhood graph comprising information on the one or moreaccess points, and propagates the information including the contextaccording to the information accessed from the neighborhood graph. 25.The access point according to claim 24, wherein the information on theone or more access points comprises information including a connectionrelationship between the post access point and the one or more accesspoints.
 26. The access point according to claim 24, wherein theneighborhood graph is provided in one of the post access point and aserver of the wireless network.
 27. The access point according to claim23, wherein each of the one or more access points is an access point towhich the station during handoff is directly associable from the accesspoint without going through another access point of the wirelessnetwork.
 28. The access point according to claim 21, wherein in responseto insufficient capacity of the storage, the post access point deletesan oldest context to store a newly received context.
 29. The accesspoint according to claim 21, wherein the post access point is a basestation of a telecommunication system.